This invention relates to reduction of Q loss in oscillators, and particularly to reduction of Q loss in wideband voltage controlled oscillators (VCOs).
Voltage controlled oscillators (VCOs) employ a resonant circuit excited by active devices. The resonant circuit often employs an inductor and variable capacitor, coupled in a resonant LC relation. Adjustment of the variable capacitor alters the LC resonant frequency, and hence the frequency of the oscillator. Wideband VCOs (those tuning xc2xc octave or more) exhibit a significant change in the resonant impedance of the LC circuit due to the changing quality factor, or Q, of the inductor with frequency and/or the changing Q of the capacitor with frequency and applied voltage.
In the design of VCOs employing bipolar, metal oxide (MOS) and gallium arsenide (GaAs) active devices, the loop gain is designed to ensure oscillation of acceptable magnitude under worst-case conditions. Under more favorable conditions, the oscillation may be so robust that the active device effectively saturates or xe2x80x9cbottoms outxe2x80x9d over an appreciable portion of the frequency cycle. When the active device bottoms out, it effectively shorts the resonant circuit to ground, reducing the Q of the resonant circuit (the ratio of reactance to loss resistance) and degrading the oscillator phase noise and jitter.
To overcome this problem, oscillators have been operated from current sources, rather than voltage sources. As a result, the average current through the oscillator is limited, and the oscillator voltage drops under more robust conditions. This technique maintains a higher Q, but it also decreases oscillation amplitude, resulting in degradation of phase noise and jitter by decreasing the ratio of the oscillation amplitude to the circuit noise sources.
The suitability of an oscillator for a given use is normally governed by the worst performance the oscillator may provide at any given frequency at which it is expected to operate. Even if an oscillator operates favorably under some conditions and/or frequency settings, the suitability of the oscillator is still measured by its worst-case performance. Consequently, there is a need to improve the worst-case performance of an oscillator to make the worst-case performance less different from better-case performance.
A circuit according to the present invention is arranged to provide output signals over a range of oscillating frequencies. The circuit includes a resonant circuit, at least one active circuit device operatively coupled to the resonant circuit to supply energy to the resonant circuit, and at least one unidirectional device coupled to the active circuit device. The unidirectional device permits current to flow between the active circuit device and the resonant circuit when the active circuit device adds energy to the resonant circuit and serves to impede draining energy from the resonant circuit due to increased output signal amplitude.